1. Field of the Invention
The present invention relates to silicone rubber compositions for coating air bags such as are used for safety purposes in vehicles. More specifically, the invention relates to silicone rubber compositions for coating air bags, which compositions have an adhesive strength capable of withstanding the shock of deployment, and especially high-temperature deployment. The invention relates in particular to silicone rubber compositions for coating air bags which, when coated on a base fabric to a thickness of 20 to 50 xcexcm, can prevent air from escaping for a period of several seconds.
2. Prior Art
Air bags are in wide use today as a safety device for protecting automobile occupants. Such air bags are generally made of a base fabric composed of a nylon woven fabric coated with chloroprene rubber. Yet, chloroprene rubber-coated base fabrics are not conducive to a lighter air bag construction. In addition, their properties tend to deteriorate over time, which is a particular concern.
Air bag base fabrics coated with silicone rubber have recently been described (see JP-A 63-78744). A silicone rubber-coated air bag base fabric provides excellent high-temperature properties, in addition to which the ability to coat the base fabric with a thin film of silicone rubber makes it possible to achieve a lightweight construction.
However, the silicone rubber does not adhere with sufficient strength to the synthetic woven fabrics such as nylon 66 used in air bags. Investigations on coating materials aimed at enhancing the adhesive strength and doing away with the use of solvents have led to the development of aqueous emulsion-type silicone coating materials (see, for example, JP No. 2513101 corresponding to U.S. Pat. No. 5,399,402, JP-A 9-208899, JP-A 9-208900) and liquid silicones.
Related art having to do with liquid silicone rubber coating materials is described in, for example, JP No. 2592021 corresponding to U.S. Pat. No. 5,877,256, JP-A 7-70923, JP-A 7-195990 corresponding to EP 663468A and JP-A 9-87585 corresponding to U.S. Pat. No. 5,789,084. These prior-art liquid silicone rubber compositions do indeed have improved adhesive strength to nylon and other base fabrics, and can be applied without the use of a solvent. However, in recent curtain-type air bags, the air pressure within the air bag must be maintained for a length of time following deployment, which is often beyond the capabilities of such prior-art compositions.
It is therefore an object of the invention to provide a silicone rubber composition for coating air bags which has excellent adhesive strength to the synthetic woven fabrics used to make air bags, outstanding folding and packing properties or flexibility, airtightness or sealability, and deployability or openability, and minimal change in properties or deterioration over time.
We have found that a certain type of silicone rubber composition, when coated onto a synthetic woven fabric of the type used in air bags, provides an air bag base fabric having excellent strength, adhesive properties and airtightness, and thus effectively achieves the foregoing object.
Accordingly, the invention provides a silicone rubber composition for coating air bags, which composition comprises (A) 100 parts by weight of an organopolysiloxane having at least two silicon-bonded alkenyl groups per molecule, (B) at least 0.5 part by weight but less than 5 parts by weight of finely divided silica, (C) 0.1 to 20 parts by weight of an adhesive component, (D) 5 to 40 parts by weight of a silicone-soluble resin containing a R3SiO1/2 unit and a SiO2 unit or RSiO3/2 unit or both (i.e., a R3SiO1/2 unit and a SiO2 unit and/or a RSiO3/2 unit), and bearing at least one alkenyl group per molecule, wherein R is a substituted or unsubstituted monovalent hydrocarbon group, (E) an organohydrogenpolysiloxane containing at least two units per molecule selected from among HR12SiO1/2, HR1SiO and HSiO3/2 units, wherein R1 is a substituted or unsubstituted monovalent hydrocarbon unit without an aliphatic unsaturated bond, in a sufficient amount to provide 0.5 to 3.0 moles of SiH groups per mole of alkenyl groups in components (A) and (D) combined, and (F) a platinum group catalyst in a sufficient amount to provide 1 to 2,000 ppm of platinum group metal atoms based on the weight of components (A) to (E) combined.